Combustors are commonly used in industrial and commercial operations to ignite fuel to produce combustion gases having a high temperature and pressure. For example, gas turbines and other turbo-machines typically include one or more combustors to generate power or thrust. A typical gas turbine used to generate electrical power includes an axial compressor at the front, multiple combustors around the middle, and a turbine at the rear. Ambient air enters the compressor as a working fluid, and the compressor progressively imparts kinetic energy to the working fluid to produce a compressed working fluid at a highly energized state. The compressed working fluid exits the compressor and flows through one or more fuel nozzles and/or tubes in the combustors where the compressed working fluid mixes with fuel before igniting to generate combustion gases having a high temperature and pressure. The combustion gases flow to the turbine where they expand to produce work. For example, expansion of the combustion gases in the turbine may rotate a shaft connected to a generator to produce electricity.
Various factors influence the design and operation of the combustors. For example, higher combustion gas temperatures generally improve the thermodynamic efficiency of the combustors. However, higher combustion gas temperatures may also promote flame holding conditions in which the combustion flame migrates toward the fuel being supplied by the fuel nozzles, possibly causing accelerated wear to the fuel nozzles in a relatively short amount of time. In addition, higher combustion gas temperatures generally increase the dissociation rate of diatomic nitrogen, increasing the production of nitrogen oxides (NOx). Conversely, a lower combustion gas temperature associated with reduced fuel flow and/or part load operation (turndown) generally reduces the chemical reaction rates of the combustion gases, increasing the production of carbon monoxide and unburned hydrocarbons.
Although effective at enabling higher operating temperatures while protecting against flame holding and controlling undesirable emissions, at particular operating conditions, some combustors may produce combustion instabilities that result from an interaction or coupling of the combustion process or flame dynamics with one or more acoustic resonant frequencies of the combustor. For example, one mechanism of combustion instabilities may occur when the acoustic pressure pulsations cause a mass flow fluctuation at a fuel port which then results in a fuel-air ratio fluctuation in the flame. When the resulting fuel/air ratio fluctuation and the acoustic pressure pulsations have a certain phase behavior (e.g., in-phase or approximately in-phase), a self-excited feedback loop results. This mechanism, and the resulting magnitude of the combustion dynamics, depends on the delay time between the injection of the fuel and the time when it reaches the flame zone, known in the art as “convective time” (Tau). Generally, there is an inverse relationship between convective time and frequency: that is, as the convective time increases, the frequency of the combustion instabilities decreases; and when the convective time decreases, the frequency of the combustion instabilities increases.
It has been observed that, in some instances, combustion dynamics may reduce the useful life of one or more combustor and/or downstream components. For example, the combustion dynamics may produce pressure pulses inside the fuel nozzles and/or combustion chambers that may adversely affect the high cycle fatigue life of these components, the stability of the combustion flame, the design margins for flame holding, and/or undesirable emissions. Alternately, or in addition, combustion dynamics at specific frequencies and with sufficient amplitudes, that are in-phase and coherent, may produce undesirable sympathetic vibrations in the turbine and/or other downstream components.
By shifting the frequency of the combustion instability in one or more combustors away from the other combustors, the coherence of the combustion system as a whole will be reduced, and the combustor-to-combustor coupling will be diminished. This selective frequency shifting reduces the ability of the combustor system tone to cause a vibratory response in downstream components and also encourages destructive interference from combustor-to-combustor, reducing the amplitudes of combustion dynamics. Therefore, a system and method that selectively adjusts the phase and/or coherence of the combustion dynamics produced by at least some of the combustors in the combustor array would be useful to enhancing the thermodynamic efficiency of the combustion system, protecting against accelerated wear, promoting flame stability, and/or reducing undesirable emissions over a wide range of operating levels.